Increase in capacity of various optical discs has been achieved by reducing size of record marks (including pits) which are binary data formed on tracks of the optical discs, and by reducing focus spot size on focal planes using an objective lens which makes a laser beam used for recording and reproduction have a shorter wavelength and has a large numerical aperture.
For example, in CDs (compact discs), a disc substrate functioning as a light transmission layer has a thickness of approximately 1.2 (mm), a laser-beam wavelength of approximately 780 (nm) is employed, a numerical aperture of an objective lens is 0.45, and a recording capacity is 650 (MB). A resolution of pits for recording signals is restricted by a diffraction limit. The diffraction limit DL is given as
DL=λ/(4×NA), in which a laser-beam wavelength λ and a numerical aperture NA are used. The diffraction limit in CDs is calculated from this equation, which yields a value of approximately 430 (nm). In CDs, since a shortest data length (shortest pit length) is approximately 830 (nm), a size of the shortest data length is approximately 1.93 times of a focus spot size determined by the diffraction limit.
Moreover, in DVDs (digital versatile discs), a light transmission layer has a thickness of approximately 0.6 (mm), a laser-beam wavelength of approximately 650 (nm) is employed, an NA is 0.6, and a recording capacity is 4.7 (GB). The diffraction limit in DVDs can be calculated from the same equation as that in the case of CDs, which yields a value of approximately 270 (nm). In DVDs, a shortest data length (shortest pit length) is approximately 400 (nm) and a size of the shortest data length is approximately 1.48 times of a focus spot size.
Furthermore, in BDs (Blu-ray discs), a light transmission layer has a thickness of 0.1 (mm), a laser-beam wavelength of approximately 405 (nm) is employed, an NA is 0.85, and a recording capacity is 25 (GB) per one recording layer. The diffraction limit in BDs can be calculated from the same equation as that in the case of CDs, which yields a value of approximately 120 (nm). In BDs, a shortest data length (shortest pit length) is approximately 150 (nm) and a size of the shortest data length is approximately 1.25 times of a focus spot size.
As described above, the increase in capacity of optical discs is achieved not only by reducing a focus spot in size but also by reducing a ratio of a size of the shortest data length (the shortest pit length) to a focus spot size (approximately 1.93 times in CDs and approximately 1.25 times in BDs). In order to reduce the ratio, it is necessary to reduce an SNR (Signal to Noise Ratio) required in reproduced signals which are read out. As a signal processing technique for this, a PRML scheme, in which a condition that reproduced waveforms from optical discs have known partial-response characteristics is combined with a maximum likelihood estimation method according to Viterbi decoding scheme, has been developed. This technique has contributed to improvement in error rates.
For example, for BDs, a PRML scheme where (1, 2, 2, 1) is used as a partial response class is commonly used. The class (1, 2, 2, 1) is an expression of optical responses to recorded binary data (intersymbol interference) in seven gradation levels (amplitude levels), and it allows an expression approximately expressing actual reproduced waveforms. In the PRML scheme, ideal optical responses which approximately expresses reproduced waveforms are derived using the maximum likelihood estimation method (Viterbi decoding scheme), thereby estimating binary data recorded on BDs.
Moreover, in HD DVDs (High-Definition Digital Versatile Disc), a shortest data length (shortest pit length) is approximately 200 (nm) and is less than a diffraction limit of approximately 270 (nm). For this reason, in a case of HD DVDs, the shortest data (shortest pit) can be read by using (1, 2, 2, 2, 1) as a partial response class and expressing an optical response (intersymbol interference) to recorded binary data in nine gradation levels (amplitude levels).
As described above, since it is difficult to physically improve a resolution which is restricted by the diffraction limit, signal processing plays a more major role in achieving the increase in capacity of optical discs. In particular, it is not expected that a laser-beam wavelength shorter than a wavelength of 405 (nm) for BDs is put to practical use, from viewpoints of inviting deteriorations in optical elements and expecting harmful effects on the human body. For this reason, it is intended to realize the increase in capacity by a method that uses nearfield light, multilayering of recording layers, use of holography or other methods. If asymmetry of a reproduced waveform is deteriorated or signal intensity near the shortest data length decreases, quality of a reproduced signal is further deteriorated and therefore a further improvement in signal processing techniques is required. Moreover, deterioration of a reproduced signal in quality harmfully influences extraction of a clock signal.
For example, Non-Patent Documents 1 and 2 disclose optical super resolution techniques called Super-RENS (Super REsolution Nearfield Structure). According to the techniques, by causing a refractive-index change at a local part where light intensity is large or a temperature is high in a focus spot on an optical disc, it is possible to reproduce record marks that are smaller than the diffraction limit λ/(4×NA) determined by a numerical aperture NA of a condenser lens which is an optical element of an optical disc apparatus and a wavelength λ of light. The local part where the refractive-index change is caused is now simply referred to as an aperture. Since this aperture is excited by energy and is derived by the refractive-index change accompanied by a crystal-structure change, there is a temporal delay in response to light. If this delay is not negligible to a rotation speed of the optical disc, a signal read out by near-field light is undesirably partially delayed, thereby producing harmful influence on decoding of a signal and extraction of a clock signal.
In optical disc apparatuses, data recorded in an optical disc itself is recorded with a stable clock signal. However, at a time of reproducing from the optical disc, it is impossible to regenerate a spindle rotation which is completely the same as that at a time of recording, and therefore it is necessary to reproduce the clock signal each time. In optical disc apparatuses, it is common to adopt a method of extracting a clock signal from a reproduced signal itself using a PLL (Phase-Locked Loop) circuit. In general, a PLL circuit is formed by a phase comparator, a loop filter and a voltage-controlled oscillator. The phase comparator compares a phase which is calculated from a reproduced signal sampled by a clock signal and a phase of the clock signal itself, thereby outputting a phase error signal corresponding to a phase difference between them. The loop filter supplies a control voltage which is obtained by filtering the phase error signal from the phase comparator, to the voltage-controlled oscillator. The voltage-controlled oscillator outputs a clock signal of a frequency proportional to the control voltage. A reproduced signal is sequentially sampled by the output clock signal from the voltage-controlled oscillator, the output clock signal from the voltage-controlled oscillator influences a calculation of a phase of a reproduced signal, and thus the PLL circuit forms a loop feedback circuit. By the loop feedback circuit, a frequency and a phase difference of the output clock signal vary in accordance with a frequency of an input signal. When a phase error between a sampling point of the reproduced signal and the clock signal is calculated, a point (a crossing point) where the reproduced signal intersects a certain slice level is defined as a clock point. In other words, a difference between the crossing point and the sampling point is a phase error between the reproduced signal (reproduced waveform) and the clock signal, and the loop feedback circuit works so as to make them equal. As a slice level, a center level (average level) of a reproduced waveform is usually used and it is a level where there are largest number of the crossing points in a reproduced waveform of an optical disc.
Moreover, there is a method of calculating a phase error between a reproduced waveform and a clock signal, in combination with a PRML scheme (see Patent Document 1, for example). This is a method of calculating a phase error at each sampling point, without setting a slice level. In this method, an ideal waveform is predicted according to the PRML scheme and a difference between the ideal waveform and the reproduced waveform is calculated as a phase error.